There have been lead-acid batteries and alkaline batteries widely used as storage battery cells for various power supplies. Among them, the alkaline storage batteries are high in reliability and can be reduced in size and weight, and hence have been used widely, the small size thereof for various portable devices and the large size in industrial applications.
In the alkaline storage batteries, positive electrodes have been made of nickel in most cases although there have been employed electrodes partly of air and silver oxide. The storage batteries have been improved in their performance as the electrodes have been changed from a pocket type to a sintered one, and have been used in wider fields as the batteries were permitted to be sealed.
On the other hand, most negative electrodes are made of cadmium at the present, although zinc, iron, hydrogen and the like besides cadmium have been considered for negative electrodes. In order to achieve a higher energy density, however, an interest has been directed to nickel-hydrogen storage batteries using metal hydrides, i.e, hydrogen storage alloys, and many proposals have been made for the production thereof.
In the alkaline storage battery employing a negative electrode made of a hydrogen storage alloy capable of reversibly absorbing and desorbing hydrogen, the hydrogen storage alloys have a theoretical capacity density greater than that of the cadmium electrode, and are free from deformation or formation of dendrites as experienced in the zinc electrode. It is expected that such hydrogen storage alloys are to be useful as a negative electrode for an alkaline storage battery possessing a high energy density, long life and free of pollution.
As the alloys used in such hydrogen storage alloy electrodes, multi-component alloys such as Ti-Ni, La-Ni, and Mm-Ni systems (Mm is a misch metal) are known.
The multi-component alloy of the Ti-Ni system may be classified as an AB type (A is an element having a high affinity for hydrogen such as La, Zr, Ti and the like; B is a transition element such as Ni, Mn, Cr and the like). This alloy is known to exhibit a relatively high discharge capacity at an initial stage of charging and discharging cycles, but it has a problem in that it is difficult to sustain the capacity for a long period of time while repeating the charging and discharging cycles.
Multi-component alloys of La-Ni or Mm-Ni systems classified as AB.sub.5 type have been intensively developed recently as materials for electrodes, and in particular, the Mm-Ni system multi-component alloys are already in commercial use. These alloys show relatively small discharge capacity, short useful life as the electrode for a storage battery, and high material cost. There has therefore been a keen demand for a novel hydrogen storage alloy material having a large discharge capacity and a long service life.
In contrast, Laves phase alloy of an AB.sub.2 type has a higher hydrogen absorbing ability, and has been considered promising for electrodes having a high capacity and long life. This alloy system has been already proposed as, for example, ZrMo.alpha.Ni.beta. alloy (Laid Open Japanese Patent Application No. 1-48370), Zr.alpha.V.beta.Ni.gamma.M.delta. alloy (Laid Open Japanese Patent Application No. 1-60961; where M is Mg, Ca, Y, Hf, Nb, etc.), AxByNiz alloy (Laid Open Japanese Patent Application No. 1-102855, where A is Zr, etc.; B is Ni, Cr, Mo, Mn, Fe, etc.), Ab.alpha. alloy (U.S. Pat. No. 4,946,646; where A is Zr, Ti, Hf, Ta, Y, etc.; B is Ni, V, Cr, Mn, Fe, etc.) and V-Ti-Zr-Ni-Cr alloy (U.S. Pat. No. 5,096,667).
When the Laves phase alloy of the AB.sub.2 type is used for electrodes, a higher discharge capacity and a possible longer life can be realized as compared with those of the multi-component alloys of the Ti-Ni system or La (or Mm)-Ni system, and a further improvement of performance is expected. The prior inventors limited the alloy system to the Zr-Mn-M-Cr/Ni system (M is one or more elements selected from V and Mo), and specified the scope of its composition, thereby succeeding in obtaining hydrogen storage alloy electrodes possessing a discharge capacity of about 350 mAh/g (U.S. Pat. No. 5,149,383). The principal phase of the alloy of the hydrogen storage alloy electrodes is a C15 type Laves phase (MgCu.sub.2 type face centered cubic structure), but phases other than the C15 type Laves phase are present in large amounts. It is therefore desireable to increase the homogeneity of the alloy, maintain the excellent cycle characteristics, and enhance the discharge capacity furthermore.
Incidentally, when a nickel-hydrogen storage battery is comprised of hydrogen storage alloy electrodes, the temperature of the storage battery is raised by hydrogenation heat of the hydrogen storage alloy in the negative electrode at the time of rapid charging. The hydrogen equilibrium pressure of the hydrogen storage alloy is also raised which causes elevation of the gas pressure in the battery, and hence the hydrogen storage capacity tends to decline.